TECHNICAL FIELD
[0001] The present invention relates to a high-strength and high-toughness magnesium based
alloy and more particularly, to a high-strength and high-toughness magnesium based
alloy that is superior in strength characteristics such as static tensile characteristics,
fatigue strength, and creep characteristics, and superior in toughness such as breaking
extension at room temperature and at high temperature up to 200 °C. Such high-strength
and high-toughness magnesium based alloy is advantageously applied to a car component
and especially to an engine part or a mission part used at a high temperature.
BACKGROUND ART
[0002] Since a magnesium alloy has low specific gravity and is light in weight, it can be
widely used in a package of a mobile phone or a portable acoustic instrument, a car
component, a machine part, a structural material and the like. Especially, in order
to maximize the effect of light in weight, it is to be employed in a part of motor
system or an operating system and more particularly, in a part of an engine system
or driving system like a piston.
[0003] However, these parts and members require heat resistance characteristics around 200°C
in addition to the strength and toughness at room temperature. According to a conventional
magnesium alloy, Mg-Al-Zn-Mn group alloy such as AZ91D alloy or Mg-Al-Mn group alloy
such as AM60B alloy defined in JIS standard, for example, since its strength is lowered
at a temperature above 120°C, it cannot be used in the above part.
[0005] Japanese Unexamined Patent Publication No. 2002-129272 discloses a Mg-Al-Zn-Ca-RE-Mn group magnesium alloy for die-casting that is superior
in creep characteristics at high temperature around 150°C. Since the magnesium alloy
disclosed in this document is manufactured by the casting method similar to the case
by Mr. Guangyin and the like, the following problems are pointed out.
- (1) The crystal grain of magnesium is as large as 60 to 150 µm.
- (2) The compound such as Al11RE3, Al2Ca, and Mg17Al12 deposited and dispersed in the matrix grows to be coarse and becomes an acicular
compound having a length of 20 to 40 µ m or more.
- (3) The acicular compound exist in a magnesium crystal grain boundary and when it
is excessively formed, it exists like a network along the boundary.
[0006] As a result, there arises a problem such that it is inferior in strength and toughness
at room temperature. Furthermore, when each element is added excessively in order
to improve the tensile characteristics at high temperature, a problem such as fluidity
or hot cracking is generated at the time of casting, so that the content of an additive
element is limited and further improvement in heat resistance characteristics is not
expected. For example, a magnesium alloy provided by die-casting disclosed in the
Japanese Unexamined Patent Publication No. 2002-129272 is defined in its appropriate content within a range containing, by weight, 1 to
3% RE, 1 to 3% Ca, and 0.5 to 8% Al.
[0007] According to a high-strength magnesium alloy and a heat treatment method of a magnesium
alloy cast disclosed in
Japanese Unexamined Patent Publication No. 8-41576, it is described that a cast alloy containing, by weight, 1 to 4% Al, 1 to 8% RE,
0.3 to 1.3% Ca, 0.1 to 2% Mn and the balance Mg has superior creep characteristics.
Furthermore, when a heat treatment such as solution treatment or ageing treatment
is performed to the Mg alloy according to need, the characteristics are improved by
enhancement of solid solution of Al or Ca and enhancement of deposition of Mg-Ca group
compound.
[0008] However, since the magnesium alloy disclosed in the
Japanese Unexamined Patent Publication No. 8-41576 is manufactured by the casting method, the Mg crystal grain is inevitably grown and
becomes coarse during its solidification. As a result, since its tensile strength
becomes 200 to 280MPa at room temperature, it cannot be applied to a car equipment
or a machine part or a structural material.
[0009] The inventor of the present invention found that the following conditions were required
to implement both high strength and high toughness (extension) of the magnesium alloy
within a temperature range from room temperature up to around 200°C.
- (1) The crystal grain diameter of a magnesium alloy that constitutes a matrix is to
be miniaturized.
- (2) A compound that is superior in heat resistance is to be uniformly deposited and
dispersed not as an acicular grain but as a fine grain.
- (3) The above compound grain is to be dispersed in a magnesium crystal grain as much
as possible.
- (4) In order to deposit and disperse the fine compound superior in heat resistance
as much as possible, it is effective to use a solid-phase (non-dissolved) manufacturing
method not using a conventional casting or die-casting method but using a plastic
forming method using powder or chips as a starting raw material.
DISCLOSURE OF THE INVENTION
[0010] The present invention has been made in view of the above findings and it is an object
of the present invention to provide a high-strength and high-toughness magnesium based
alloy that is superior in tensile strength, breaking extension and fatigue strength
at room temperature and at the same time has high heat resistance characteristics
at around 200°C.
[0011] It is another object of the present invention to provide a manufacturing method of
a high-strength and high-strength magnesium based alloy material having the above
superior characteristics.
[0012] A high-strength and high-toughness magnesium based alloy according to the present
invention contains, by weight, 1 to 8% rare earth element and 1 to 6% calcium, and
characterized in that the maximum crystal grain diameter of magnesium that constitutes
a matrix is not more than 30µ m.
[0013] Preferably, the magnesium based alloy contains at least one intermetallic compound
of the rare earth element and the calcium, in which the maximum grain diameter of
the intermetallic compound is not more than 20µ m. One example of the intermetallic
compound is a compound of aluminum and rare earth element. Another example of the
intermetallic compound is a compound of aluminum and calcium.
[0014] When it is assumed that the maximum grain diameter of the intermetallic compound
is "D" and the minimum grain diameter thereof is "d", D/d ≦ 5 is satisfied. Further
preferably, the intermetallic compound is dispersed in the crystal grain boundary
and crystal grain of magnesium that constitutes the matrix. Here, the maximum grain
diameter means the maximum length of the compound grain and the minimum grain diameter
means the minimum length of the compound grain.
[0015] Preferably, the maximum crystal grain diameter of magnesium that constitutes the
matrix is not more than 20µm. More preferably, it is not more than 10 µm.
[0016] According to one embodiment, the high-strength and high-toughness magnesium based
alloy contains at least one kind of element selected from a group consisting of, by
weight, 0.5 to 6% Zinc, 2 to 15% aluminum, 0.5 to 4% manganese, 1 to 8% silicon and
0.5 to 2% silver.
[0017] Focusing on the mechanical characteristics of the high-strength and high-toughness
magnesium based alloy according to the present invention, a tensile strength (σ) is
not less than 350MPa and a breaking extension (ε) is not less than 5%. In addition,
focusing on another viewpoint, the product of the tensile strength (σ) and the breaking
extension (ε) is such that σ × ε ≧ 4000MPa ·%.
[0018] Preferably, the rare earth element contains at least one kind of element selected
from a group consisting of cerium (Ce), lanthanum (La), yttrium (Y), ytterbium (Yb),
gadolinium (Gd), terbium (Tb), scandium (Sc), samarium (Sm), praseodymium (Pr), and
neodymium (Nd).
[0019] According to one embodiment, the high-strength and high-toughness magnesium based
alloy contains, by weight, 1.5 to 4% manganese, 2 to 15% aluminum and iron of 10ppm
or less and the maximum grain diameter of an Al-Mn compound is not less than 20 µ
m. Here, it is to be noted that the term "iron of 10ppm or less includes that iron
is not included.
[0020] According to the high-strength and high-toughness magnesium based alloy comprising
the above constitution, since there is provided a structure in which the matrix comprises
magnesium having fine crystal grain diameter and the fine granular intermetallic compound
is uniformly deposited and dispersed in the crystal grain, it can be advantageously
applied to a driving system part for a car or a two-wheeled motor vehicle.
[0021] A manufacturing method of the high-strength and high-toughness magnesium based alloy
material according to the present invention comprises the following steps.
- (1) A step of miniaturizing a magnesium crystal grain that constitutes a matrix and
miniaturizing a compound grain dispersed in the matrix by performing a plastic forming
process to magnesium based alloy powder containing, by weight, 1 to 8% rare earth
element and 1 to 6% calcium.
- (2) A step of manufacturing a powder solidified body from the miniaturized magnesium
based alloy powder by compression molding.
- (3) A step of providing an alloy material by heating the powder solidified body and
immediately performing a warm extrusion process to it.
[0022] The working effect of the above-described present invention will be described in
the following "BEST MODE FOR CARRYING OUT INVENTION" and "EXAMPLES")
BRIEF DESCRIPTION OF DRAWINGS
[0023]
Fig. 1 is a view schematically showing the crystal structure of a magnesium based
alloy manufactured by a casting method;
Fig. 2 is a view schematically showing the crystal structure of a magnesium based
alloy manufactured by a solid-phase manufacturing method using a plastic forming method;
Fig. 3 is a view showing manufacturing steps of a high-strength and high-toughness
magnesium based alloy material according to the present invention;
Fig. 4 is a view showing one example of steps performed repeatedly for starting raw
material powder until a powder solidified body is finally obtained;
Fig. 5A shows a structure photograph of a working example 9 shown in Table 1;
Fig. 5B shows a structure photograph of a working example 11 shown in Table 1;
Fig. 5C shows a structure photograph of a comparative example 16 shown in Table 1;
Fig. 6A shows a structure photograph of an extruded material (working example); and
Fig. 6B shows a structure photograph of an extruded material (comparative example).
BEST MODE FOR CARRYING OUT THE INVENTION
[Effect of each additive element]
(1) Rare earth (RE) element
[0024] A rare earth (RE) element component forms a Mg-RE compound with magnesium that is
a matrix, and forms Al-RE compound with aluminum (Al) that is an example of an additive
component. Since the compound such as Al
2RE or Al
11RE
3 is superior in heat stability as compared with Mg-Al group compound such as Mg
2Al
3 or Mg
17Al
12, when its fine powder is diffused uniformly in the matrix, the heat resistance characteristics
of a magnesium alloy can be improved.
[0025] The appropriate range of a rare earth (RE) element content is 1 to 8% by weight.
When the rare earth (RE) element content is less than 1%, the heat resistance characteristics
are not sufficiently improved. Meanwhile, when the rare earth (RE) element content
is more than 8%, the effect is not increased and on the contrary, the deposited amount
of the compound becomes excessive, which causes a problem in the subsequent process.
That is, when a secondary process such as warm forging, rolling or drawing is performed
for the provided magnesium alloy, cracking is generated due to lack of toughness.
A more preferable rare earth element content to provide both high strength and high
toughness and preferable secondary process workability is 3 to 5%.
[0026] By a normal casting method or a die-casting method, the Mg-RE group compound and
the Al-RE group compound are deposited along a crystal grain boundary (α crystal grain
boundary) of magnesium and exist as acicular compounds or network-like compounds formed
with the connected acicular compounds as shown in Fig. 1.
[0027] Fig. 1 is a view schematically showing the crystal structure of a magnesium based
alloy manufactured by a casting method. A magnesium crystal grain 1 that constitutes
a matrix is coarse and an acicular intermetallic compound 3 is provided along a crystal
grain boundary 2. When the acicular intermetallic compound 3 exists along the crystal
grain boundary 2 of the matrix in this way, the mechanical characteristics of the
magnesium based alloy is lowered.
[0028] In view of the improvement in strength and toughness of the magnesium based alloy,
it is desirable that the intermetallic compound is dispersed in the crystal grain
as a fine granular compound. Fig. 2 is a view schematically showing the crystal structure
of a magnesium based alloy manufactured by a method of the present invention that
will be described below, that is, a solid-phase manufacturing method using a plastic
forming method. A magnesium crystal grain 4 that constitutes a matrix is fine and
a fine granular intermetallic compound 6 is dispersed in a crystal grain boundary
5 and the crystal grain 4. The magnesium based alloy having the above structure provides
superior characteristics in strength and toughness.
[0029] Regarding the size of the intermetallic compound, a maximum grain diameter is preferably
not more than 20µ m in view of providing both high strength and high toughness, and
more preferably, it is not more than 10 µ m. When the maximum grain diameter of the
intermetallic compound is more than 20 µm, the toughness (breaking extension or an
impact resistance value) of the magnesium alloy at room temperature is lowered and
especially when it is more than 30µ m, the strength is lowered with the lowering of
the toughness.
[0030] Regarding the configuration of the intermetallic compound, it is more preferably
granular than acicular. More specifically, when it is assumed that the maximum grain
diameter of the compound grain is "D" and the minimum grain diameter thereof is "d",
by making an aspect ratio D/d below 5, both high strength and high toughness can be
provided. In view of the improvement of fatigue strength, it is more preferably made
below 3. Meanwhile, when the ratio D/d is more than 5, the magnesium alloy becomes
defective and since a stress is concentrated at that part, the toughness is lowered.
[0031] Since the ratio D/d of the acicular compound deposited along the α crystal grain
boundary by the casting method or die-casting method is 5 to 20, it is difficult to
provide high strength and high toughness, and it is also difficult to provide high
fatigue strength.
[0032] In addition, as the rare earth element, cerium (Ce), lanthanum (La), yttrium (Y),
ytterbium (Yb), gadolinium (Gd), terbium (Tb), scandium (Sc), samarium (Sm), praseodymium
(Pr), neodymium (Nd) and the like may be used. In addition, a misch metal containing
the above rare earth element may be used.
(2) Calcium (Ca)
[0033] Calcium (Ca) forms an Al-Ca group compound such as Al
2Ca with aluminum (Al) that is one example of the additive component. Since this intermetallic
compound is superior in heat stability as compared with the Mg-Al group compound such
as Mg
2Al
3 or Mg
17Al
12 similar to the above Al-RE group compound, when its fine compound grains are uniformly
dispersed in the matrix, the heat resistance characteristics of the magnesium alloy
can be improved. In addition, when Zn is contained, a Mg-Zn-Ca group compound is formed
and this contributes to the improvement of the heat resistance characteristics similar
to Al
2Ca.
[0034] An appropriate calcium content is 1 to 6% by weight. When the calcium content is
less than 1%, the effect of the improvement of the heat resistance characteristics
is not sufficiently provided. Even when the calcium content is more than 6%, the effect
is not increased and on the contrary, the deposited amount of the compound becomes
excessive and a problem is raised in the subsequent process. That is, when a secondary
process such as warm forging, rolling or drawing is performed for the provided magnesium
alloy, cracking is generated due to lack of toughness. A more preferable calcium content
to provide both high strength and high toughness and preferable secondary process
workability is 2 to 5%.
[0035] By a normal casting method or a die-casting method, the Al-Ca group compound and
the Mg-Zn-Ca group compound are also deposited along a crystal grain boundary (α crystal
grain boundary) of magnesium and exist as acicular compounds or network-like compounds
formed with the connected acicular compounds. As a result, the mechanical characteristics
of the magnesium based alloy is lowered. Hence, according to the present invention,
as described above, by applying strong processing strain when a powdered or aggregated
starting raw material is solidified by the plastic forming method, the acicular or
network-like Al-Ca group compound and Mg-Zn-Ca group compound are finely ground and
uniformly dispersed in the magnesium crystal grain boundary and the magnesium crystal
grain as shown in Fig. 2.
[0036] Regarding the size of the intermetallic compound, a maximum grain diameter is preferably
not more than 20µm in view of providing both high strength and high toughness, and
more preferably, it is not more than 10 µ m. When the maximum grain diameter of the
intermetallic compound is more than 20 µm, the toughness (breaking extension or an
impact resistance value) of the magnesium alloy at room temperature is lowered and
especially when it is more than 30 µm, the strength is lowered with the lowering of
the toughness.
[0037] Regarding the configuration of the intermetallic compound, it is more preferably
granular than acicular. More specifically, when it is assumed that the maximum grain
diameter of the compound grain is "D" and the minimum grain diameter thereof is "d",
by making an aspect ratio D/d below 5, both high strength and high toughness can be
provided. In view of the improvement of fatigue strength, it is more preferably made
below 3. Meanwhile, when the ratio D/d is more than 5, the magnesium alloy becomes
defective and since a stress is concentrated at that part, the toughness is lowered.
Since the ratio D/d of the acicular compound deposited along the α crystal grain boundary
by the casting method or die-casting method is 5 to 20, it is difficult to provide
high strength and high toughness, and it is also difficult to provide high fatigue
strength.
(3) Aluminum (Al)
[0038] Aluminum forms a Mg-Al group compound with magnesium of the matrix and forms a Mg-Zn-Al
group compound. Since the latter is superior in heat resistance, when it is deposited
and finely dispersed in the matrix, it contributes to the improvement of the heat
resistance characteristics of the magnesium alloy. In order to provide such effect,
an Al content has to be not less than 2% by weight. Meanwhile, when the Al content
is more than 15%, a crack is generated in an ingot in the course of manufacturing
the ingot, causing the productivity and yield to be lowered. Therefore, the appropriate
content of the Al component in the magnesium alloy in the present invention is preferably
in a range of 2 to 15% and in view of providing both high strength and high toughness
and the above preferable secondary process workability, it is more preferably in a
range of 6 to 12%.
(4) Zinc (Zn)
[0039] Although zinc forms a Mg-Zn compound with magnesium of the matrix, since this two-element
compound is inferior in heat stability, it lowers the heat resistance characteristics
of the magnesium alloy. However, as described above, when Al is added, a Mg-Zn-Al
group compound or Mg-Zn-Ca group compound that is superior in heat resistance is formed
and when solid solution hardens the matrix as will be described below, it contributes
to the improvement of the heat resistance characteristics and mechanical characteristics
of the magnesium alloy at room temperature. An appropriate Zn content in the magnesium
alloy in the present invention is 0.5 to 6% by weight. When it is less than 0.5%,
the above effect is not sufficiently provided but when it is more than 6%, the toughness
of the magnesium alloy is lowered.
(5) Manganese (Mn)
[0040] Manganese (Mn) becomes solid solution in the magnesium matrix and it contributes
to the improvement of the mechanical characteristics and especially resistance because
of solid solution hardening. An appropriate Mn content in the magnesium alloy in the
present invention is 0.5 to 4% by weight. When it is less than 0.5%, the above effect
cannot be sufficiently provided but when it is more than 4%, the toughness of the
magnesium alloy is lowered.
[0041] When the Mn content is 1.5 to 4%, a Fe content in the magnesium based alloy is preferably
not more than 10ppm and more preferably not more that 3ppm, and at the same time the
maximum grain diameter of the Al-Mn compound is preferably not more than 20µ m and
more preferably not more than 10 µm.
[0042] When a lot of Mn is added, the Fe content that lowers corrosion resistance is reduced
in the cast magnesium ingot, so that corrosion resistance of the magnesium alloy is
improved. However, when Mn is added excessively (1% or more, for example), the Al-Mn
compound becomes coarse (about 20 to 80 µm, for example), which lowers the mechanical
characteristic and processability of the magnesium alloy.
[0043] However, when a mechanical grinding and miniaturizing process according to the present
invention that will be described below is used, the above described structure in which
the maximum grain diameter of the Al-Mn compound is not more than 20 µ m and more
preferably not more than 10 µ m can be implemented, so that the magnesium based alloy
can provide balanced corrosion resistance and mechanical characteristics.
(6) Silver (Ag)
[0044] Silver (Ag) becomes solid solution in the magnesium matrix and it contributes to
the improvement of the mechanical characteristics and especially resistance because
of solid solution hardening. An appropriate Ag content in the magnesium alloy in the
present invention is 0.5 to 2% by weight. When it is less than 0.5%, the above effect
cannot be sufficiently provided but when it is more than 2%, the toughness of the
magnesium alloy is lowered.
(7) Silicon (Si)
[0045] Silicon (Si) forms magnesium silicide (Mg
2Si) with magnesium of the matrix. Since this magnesium silicide has high rigidity,
high hardness and high corrosion resistance, when it is dispersed in the matrix, the
above characteristics in the magnesium alloy can be improved also. When a Si content
is less than 1% by weight, this effect is not sufficient but when it is more than
8%, the toughness of the magnesium alloy, extension in the tensile characteristics
especially is considerably lowered and at the same time, tool abrasion in the cutting
process is generated and material surface roughness is lowered associated with it.
[Maximum crystal grain diameter of magnesium of matrix]
[0046] According to the magnesium alloy of the present invention, both strength and toughness
can be improved by miniaturizing the magnesium crystal grain that constitutes the
matrix. More specifically, it has been found that when the maximum crystal grain diameter
of magnesium is not more than 30
µm, the magnesium alloy has high strength and high toughness such that tensile strength
is not less than 350MPa and breaking extension is not less than 5% at room temperature.
Especially, when the maximum crystal grain diameter is not less than 20 µ m, the magnesium
alloy has high strength above 400MPa. Furthermore, it has been found that when the
maximum crystal grain diameter of magnesium is below 10 µ m, during the process of
plastic forming of Mg raw material powder, since its texture is disordered, the Mg
alloy provides high toughness and improves its bending and pressing processability
at low temperature. [Manufacturing method of high-strength and high-toughness magnesium
based alloy material]
[0047] Fig. 3 shows manufacturing steps of a high-strength and high-toughness magnesium
based alloy material according to the present invention. The method of the present
invention will be described in detail with reference to Fig. 3.
(1) Preparation of raw material powder
[0048] A magnesium alloy ingot having a predetermined component composition is manufactured
by the casting method. The predetermined component composition contains, by weight,
1 to 8% rare earth element and 1 to 6% calcium and according to need, it further contains
at least one kind selected from an element group consist of, by weight, 0.5 to 6%
zinc, 2 to 15% aluminum, 0.5 to 4% manganese, 1 to 8% silicon, and 0.5 to 2% silver.
[0049] Then, powder, aggregated grain, chip and the like is provided from the magnesium
alloy ingot manufactured by the casting method through a machining process such as
cutting or grinding process, and used as starting raw material powder.
(2) Miniaturization of crystal grain and miniaturization of compound grain
[0050] Prior to manufacturing of powder solidified body, a plastic forming process such
as compression molding, extruding, casting, or rolling is performed for the starting
material powder to miniaturize the magnesium crystal grain that constitutes the matrix
and miniaturize the compound grain dispersed in the matrix to provide a crystal structure
shown in Fig. 2.
[0051] When strong processing strain is applied to the starting raw material, the acicular
or network-like intermetallic compound (for example, Mg-RE group compound or Al-RE
group compound) can be finely ground and uniformly dispersed in the magnesium crystal
grain that constitutes the matrix.
[0052] As the method applying the strong processing strain to the magnesium alloy raw material
powder, a method in which compression molding or extruding are performed or a shearing
process, bending process, rotation shearing process and the like are performed for
the powder in a mold and the like, or a method of rolling the powder, or a method
in which a grinding process is performed with a ball mill and the like are effective.
In order to effectively miniaturize the intermetallic compound and the magnesium crystal
grain, the plastic forming method is preferably performed in a warm region at 100
to 300°C .
[0053] Fig. 4 shows one example of the processes in which the plastic forming processes
are repeatedly performed for starting raw material powder 10 until a powder solidified
body 20 is finally provided. One example of the method to apply the strong processing
strain will be described with reference to Fig. 4.
[0054] First, as shown in Fig. 4(a), a container comprising a mold mill 11 and a lower punch
12 is filled with the powder 10. Then, as shown in Fig. 4(b), a compression upper
punch 13 is lowered in the mold mill 11 to compress the raw material powder 10. Then,
as shown in Fig. 4(c) and 4(d), after the compression upper punch 13 has been retreated,
an indenting upper punch 14 is inserted into the compressed raw material powder 10.
The compressed raw material powder 10 is extruded backward (a direction shown by an
arrow B in Fig. 4) by the indenting upper punch 14 and receives strong processing
strain.
[0055] Then, as shown in Fig. 4(e) and 4(f), after the indenting upper punch 14 has been
retreated, the compressed raw material powder 10 having a U-shaped section is compressed
by the compression upper punch 13 again. The raw material powder 10 existing along
the inner wall surface of the mold mill 11 is moved inwardly (direction shown by an
arrow C in Fig. 4) in the mold mill 1 by the above compression.
[0056] A series of processes as shown in Fig. 4(b) to 4(f) is repeated to mechanically grind
the raw material powder and miniaturize the magnesium crystal grain of the matrix.
At the same time, the intermetallic compound is also finely ground and dispersed in
the magnesium crystal grain.
(3) Manufacturing of powder solidified body
[0057] As shown in Fig. 4(g), after the miniaturizing process by performing the necessary
plastic forming process to the magnesium based alloy raw material powder 10, the powder
solidified body 20 is manufactured by compression molding.
(4) Heating and warm extruding
[0058] For example, the powder solidified body provided as described above is heated up
to 300 to 520°C and maintained for 30 seconds and immediately processed by a warm
extrusion process under a condition that an extrusion rate is 37 and a mold temperature
is 400°C to be a rod-like material. The above warm extrusion process promotes the
miniaturization of the magnesium crystal grain and the compound grain. More specifically,
the compound grain is mechanically cut and further miniaturized by the plastic process
using the extrusion process, and the magnesium crystal grain is dynamically recrystallized
and further miniaturized through the process and the heat treatment.
[Mechanical characteristics of magnesium based alloy]
[0059] Since the magnesium based alloy according to the present invention is superior in
strength and toughness within a temperature range from room temperature to about 200°C
, it can be used as an engine part or a transmission part of a car or a two-wheeled
motor vehicle. When the magnesium alloy contains the above appropriate component element
defined by the present invention, and the matrix magnesium has the crystal grain diameter
that satisfies the appropriate range, the tensile strength (σ) of 350MPa or more and
the breaking extension (ε) of 5% or more at room temperature are implemented. More
preferably, the tensile strength is 400MPa or more. In addition, the magnesium alloy
has high strength and high toughness in which the product of the tensile strength
(σ) and the breaking extension (ε) is such that σ × ε ≧ 4000MPa · % .
[0060] Meanwhile, when the magnesium based alloy has the tensile strength (σ) of 350MPa
or more and breaking extension (ε) of 5% or more at room temperature and/or satisfies
that σ × ε ≧ 4000MPa ·%, it can be used as a driving part used in a car or a two-wheeled
motor vehicle such as a piston, a cylinder liner, a con-rod and the like
EXAMPLE 1
[0061] Magnesium based alloy powder (grain diameter : 0.5 to 2mm) having the alloy composition
shown in Table 1 was prepared and a mold was filled with it and then a powder solidified
body was manufactured by compression molding. This solidified body was maintained
at 400 to 480 °C for 5 minutes in an inert gas atmosphere and then immediately a warm
extrusion process was performed for it to provide an extruded material (diameter :
7.2mm φ).
[0062] The structure in the extruded direction of the above material was observed after
polishing and chemical etching and the maximum crystal grain diameter of magnesium
of the matrix was measured by image analysis. In addition, a round rod extensile test
piece (diameter : 3mm φ and parallel part : 15mm) was obtained from the extruded material
and tested at room temperature and 150 °C. The tensile speed was kept constant at
0.3mm/min and in the tensile test at 150°C, a test piece was heated and maintained
at 150°C for 100 hours before the test and tested.
[0063] These characteristic evaluation results are shown in Table 1. Regarding the crystal
grain miniaturization of the matrix, while the magnesium based alloy powder was heated
and maintained at 100 to 300°C, a plastic forming process (compression, extrusion,
shearing process and the like) was performed by press molding or rolling, and magnesium
based alloy powder having different crystal grain diameters was manufactured. In addition,
according to a comparative example 19, an extruded material was heat treated at 400°C
for 20 hours in an inert gas atmosphere to coarse the crystal grain.
[0064] According to the inventive examples 1 to 11, each extruded material has the appropriate
alloy composition and appropriate Mg maximum crystal grain diameter defined by the
present invention, so that it has superior mechanical characteristics at room temperature.
Especially, when the maximum crystal grain diameter Mg is below 10 µm as shown in
the inventive examples 10 and 11, the extension (toughness) is improved as well as
strength.
[0065] Meanwhile, according to the comparative examples 12 to 18, since the extruded material
does not have the alloy composition defined by the present invention, it does not
have enough strength. Especially, in the comparative examples 14 and 15, since a RE
or Ca content exceeds an appropriate range, the toughness is lowered and as a result,
the tensile strength is also lowered. According to the comparative example 19, since
the Mg maximum crystal grain diameter is as large as 66.8 µ m, the strength characteristics
are not sufficiently provided.
EXAMPLE 2
[0066] The structure photographs of the inventive examples 9 and 11 and the comparative
example 16 shown in Table 1 are shown in Fig. 5. It is clearly found by observing
and comparing those structure photographs that the magnesium crystal grains of the
extruded materials of the inventive examples 9 and 11 are miniaturized.
EXAMPLE 3
[0067] An ingot containing, by weight, 3.5% RE, 1.5% CA, 0.8% Zn, 7% of Al, 0.5% Mn, and
the balance Mg was manufactured by a casting method and a magnesium based alloy powder
(grain diameter 0.5 to 1.5mm) was obtained from the material. This Mg alloy powder
was heated up to 150°C and rolled to miniaturize the powder Mg crystal grain and miniaturize
the compound dispersed in the matrix. The Mg alloy powder after such warm plastic
forming process was solidified by molding and heated up and maintained at 420°C for
5 minutes in an inert gas atmosphere and then immediately a warm extrusion process
(extrusion ratio : 20) was performed for it.
[0068] Meanwhile, according to the comparative example, Mg alloy powder provided by a cutting
process without the above rolling process was directly formed by molding and it is
processed by heating and warm extrusion process in the same condition to be an extruded
material. According to the inventive example, the tensile strength of the extruded
material was 397MPa and the breaking extension thereof was 11.4% at room temperature,
while according to the comparative example, the tensile strength of the extruded material
was 316MPa and the breaking extension thereof was 6.5%.
[0069] The structures of those extruded materials are shown in Fig. 6. According to the
inventive example shown in Fig. 6(a), the compound (here, Al
2Ca and Mg
17Al
12) dispersed in the matrix has a spherical shape or a shape close to it and uniformly
dispersed in the grain boundary and the grain of the Mg crystal grain. As a result
of image analysis, the ratio (D/d) of the maximum grain diameter "D" to the minimum
grain diameter "d" of the compound is 1.2 to 2.4 and the maximum grain diameter is
3.8
µm.
[0070] Meanwhile, according to the comparative example shown in Fig. 6(b), a network-like
compound (Al
2Ca and Mg
17Al
12) connected along the Mg crystal grain boundary exists and as a result of the similar
image analysis, it is found that the intermetallic compound is coarse and have a high
D/d value exceeding 10 and its longest diameter is more than 30 µm.
EXAMPLE 4
[0071] Magnesium based alloy powder (grain diameter : 0.5 to 2mm) having the alloy composition
of each of samples No. 1 to 4 and 8 shown in Table 2 was prepared and each powder
was heated up to about 150°C to be processed by shearing and compression processes
so that the Mg crystal grain and the deposited and dispersed compound in the powder
material were miniaturized. Then, a mold was filled with the powder and then a powder
solidified body was manufactured by compression molding. This solidified body was
maintained at 400°C for 5 minutes in an inert gas atmosphere and then immediately
a warm extrusion process was performed for it to provide an extruded material (diameter
: 7.2mm φ).
[0072] Magnesium based alloys of the samples 5 to 7 are ingot materials manufactured by
the casting method.
[0073] The structure in the extruded direction of the above material was observed after
polishing and chemical etching and the maximum crystal grain diameter of the Mg matrix
and the maximum grain diameter of the Al-Mn group compound were measured by image
analysis.
[0074] In addition, a round rod extensile test piece (diameter : 3mm φ and parallel part
: 15mm) was obtained from the extruded material and tested at room temperature and
150°C. The tensile speed was kept constant at 0.3mm/min.
[0075] Furthermore, in order to evaluate the corrosion resistance of each sample, a pillar
sample having a diameter of 6.8mm φ and a length of 80mm was obtained from the extruded
material and this was immersed in NaCl aqueous solution having a concentration of
5% and pH 10 (solution temperature; 35°C) for 72 hours and its corrosion speed (mg/cm
2) was calculated from a reduced weight amount before and after the test. These characteristic
evaluation results are shown in Table 2.
[0076] According to each of the inventive examples 1 to 4, the extruded material has the
appropriate alloy composition and appropriate Mg maximum crystal grain diameter defined
by the present invention, so that each has superior mechanical characteristics and
corrosion resistance at room temperature. Especially, as the Mn content is increased
within a range of 1.5% or more, the Fe content in the Mg alloy is decreased and as
a result, the corrosion resistance is improved (corrosion speed is lowered). In addition,
the tensile strength is increased as the Mn content is increased, which is because
the dispersion of the Al-Mn group compound miniaturized to 10 µ m or less is enhanced.
[0077] Meanwhile, according to the comparative examples 5 to 7, since the extruded material
was manufactured by the casting method and does not have the Mg crystal grain diameter
defined by the present invention, it does not have enough strength. At the same time,
since the Al-Mn group compound becomes coarse such that its grain diameter is beyond
30 µm, which is one factor causing the strength and toughness of the Mg alloy to be
lowered.
[0078] Meanwhile, according to the comparative example 8, although it has a Mg crystal grain
diameter of 20
µm or less and have superior mechanical characteristics, since it does not contain
Mn, a Fe content is increased to 135ppm. As a result, the corrosion resistance of
the Mg alloy is considerably lowered.
[0079] Although the embodiments of the present invention have been described with reference
to the drawings in the above, the present invention is not limited to the above-illustrated
embodiments. Various kinds of modifications and variations may be added to the illustrated
embodiments within the same or equal scope of the present invention.
INDUSTRIAL APPLICABILITY
[0080] The present invention is applied to a magnesium based alloy having superior strength
characteristics and superior toughness at room temperature and at a high temperature
up to 200°C. Especially, since a high-strength and high-toughness magnesium based
alloy according to the present invention comprises a magnesium matrix having a fine
crystal grain diameter and has a structure in which a fine granular intermetallic
compound is uniformly deposited and dispersed in its crystal grain, it can be advantageously
applied to an engine or a driving part of a car or a two-wheeled motor vehicle.
1. A high-strength and high-toughness magnesium based alloy containing, by weight, 1
to 8% rare earth element and 1 to 6% calcium, characterized in that the maximum crystal grain diameter of magnesium that constitutes a matrix is not
more than 30 µ m.
2. The high-strength and high-toughness magnesium based alloy according to claim 1, containing
at least one intermetallic compound of said rare earth element and said calcium, wherein
the maximum grain diameter of the intermetallic compound is not more than 20 µm.
3. The high-strength and high-toughness magnesium based alloy according to claim 2, wherein
said intermetallic compound is a compound of aluminum and rare earth element.
4. The high-strength and high-toughness magnesium based alloy according to claim 2, wherein
said intermetallic compound is a compound of aluminum and calcium.
5. The high-strength and high-toughness magnesium based alloy according to claim 2, wherein
when it is assumed that the maximum grain diameter of said intermetallic compound
is "D" and the minimum grain diameter thereof is "d", D/d ≦ 5 is satisfied.
6. The high-strength and high-toughness magnesium based alloy according to claim 2, wherein
said intermetallic compound is dispersed in the crystal grain boundary and crystal
grain of magnesium that constitutes said matrix.
7. The high-strength and high-toughness magnesium based alloy according to claim 1, wherein
the maximum crystal grain diameter of magnesium that constitutes the matrix is not
more than 20 µm.
8. The high-strength and high-toughness magnesium based alloy according to claim 1, wherein
the maximum crystal grain diameter of magnesium that constitutes the matrix is not
more than 10 µm.
9. The high-strength and high-toughness magnesium based alloy according to claim 1, containing
at least one kind of element selected from a element group consisting of, by weight,
0.5 to 6% Zinc, 2 to 15% aluminum, 0.5 to 4% manganese, 1 to 8% silicon and 0.5 to
2% of silver.
10. The high-strength and high-toughness magnesium based alloy according to claim 1, wherein
a tensile strength (σ) is not less than 350MPa and a breaking extension (ε) is not
less than 5%.
11. The high-strength and high-toughness magnesium based alloy according to claim 1, wherein
the product of the tensile strength (σ) and the breaking extension (ε) is such that
σ × ε ≧ 4000MPa·%.
12. The high-strength and high-toughness magnesium based alloy according to claim 1, wherein
said rare earth element contains at least one kind of element selected from a group
consisting of cerium (Ce), lanthanum (La), yttrium (Y), ytterbium (Yb), gadolinium
(Gd), terbium (Tb), scandium (Sc), samarium (Sm), praseodymium (Pr), and neodymium
(Nd).
13. The high-strength and high-toughness magnesium based alloy according to claim 1, containing,
by weight, 1.5 to 4% manganese, 2 to 15% aluminum and iron of 10ppm or less, wherein
the maximum grain diameter of an Al-Mn compound is not less than 20µm.
14. A driving system part for a car or a two-wheeled motor vehicle using the high-strength
and high-toughness magnesium based alloy according to claim 1.
15. A manufacturing method of a high-strength and high-toughness magnesium based alloy
material comprising: a step of miniaturizing a magnesium crystal grain that constitutes
a matrix and miniaturizing a compound grain dispersed in the matrix by performing
a plastic forming process to magnesium based alloy powder containing, by weight, 1
to 8% rare earth element and 1 to 6% calcium;
a step of manufacturing a powder solidified body from said miniaturized magnesium
based alloy powder by compression molding; and
a step of providing an alloy material by heating said powder solidified body and immediately
performing a warm extrusion process to it.
Amended claims under Art. 19.1 PCT
1. (After amendment) A high-strength and high-toughness magnesium based alloy provided
such that a plastic forming process is performed to magnesium based alloy powder containing,
by weight, 1 to 8% rare earth element and 1 to 6% calcium to miniaturize a magnesium
crystal grain that constitutes a matrix and a compound grain dispersed in the matrix,
and immediately after heating a powder solidified body provided from the miniaturized
magnesium based alloy powder, a warm extrusion process is performed to it, characterized in that the maximum crystal grain diameter of magnesium that constitutes the matrix is not
more than 30 µm,
said magnesium based alloy contains at least one intermetallic compound of said rare
earth element and said calcium, and
when it is assumed that the maximum grain diameter of said intermetallic compound
is "D" and the minimum grain diameter thereof is "d", D/d ≦ 5 is satisfied.
2. (After amendment) The high-strength and high-toughness magnesium based alloy according
to claim 1, wherein the maximum grain diameter of the intermetallic compound is not
more than 20 µ m.
3. The high-strength and high-toughness magnesium based alloy according to claim 2, wherein
said intermetallic compound is a compound of aluminum and rare earth element.
4. The high-strength and high-toughness magnesium based alloy according to claim 2, wherein
said intermetallic compound is a compound of aluminum and calcium.
5. (Deleted)
6. The high-strength and high-toughness magnesium based alloy according to claim 2, wherein
said intermetallic compound is dispersed in said crystal grain boundary and crystal
grain of magnesium that constitutes said matrix.
7. The high-strength and high-toughness magnesium based alloy according to claim 1, wherein
the maximum crystal grain diameter of magnesium that constitutes the matrix is not
more than 20 µm.
8. The high-strength and high-toughness magnesium based alloy according to claim 1, wherein
the maximum crystal grain diameter of magnesium that constitutes the matrix is not
more than 10 µ m.
9. The high-strength and high-toughness magnesium based alloy according to claim 1, containing
at least one kind of element selected from a element group consisting of, by weight,
0.5 to 6% Zinc, 2 to 15% aluminum, 0.5 to 4% manganese, 1 to 8% silicon and 0.5 to
2% of silver.
10. The high-strength and high-toughness magnesium based alloy according to claim 1, wherein
a tensile strength (σ) is not less than 350MPa and a breaking extension (ε) is not
less than 5%.
11. The high-strength and high-toughness magnesium based alloy according to claim 1, wherein
the product of the tensile strength (σ) and the breaking extension (ε) is such that
σ × ε ≧ 4000MPa ·%.
12. The high-strength and high-toughness magnesium based alloy according to claim 1, wherein
said rare earth element contains at least one kind of element selected from a group
consisting of cerium (Ce), lanthanum (La), yttrium (Y), ytterbium (Yb), gadolinium
(Gd), terbium (Tb), scandium (Sc), samarium (Sm), praseodymium (Pr), and neodymium
(Nd).
13. The high-strength and high-toughness magnesium based alloy according to claim 1, containing,
by weight, 1.5 to 4% manganese, 2 to 15% aluminum and iron of 10ppm or less, wherein
the maximum grain diameter of an Al-Mn compound is not less than 20 µm.
14. A driving system part for a car or a two-wheeled motor vehicle using the high-strength
and high-toughness magnesium based alloy according to claim 1.
15. A manufacturing method of a high-strength and high-toughness magnesium based alloy
material comprising:
a step of miniaturizing a magnesium crystal grain that constitutes a matrix and miniaturizing
a compound grain dispersed in the matrix by performing a plastic forming process to
magnesium based alloy powder containing, by weight, 1 to 8% rare earth element and
1 to 6% calcium;
a step of manufacturing a powder solidified body from said miniaturized magnesium
based alloy powder by compression molding; and
a step of providing an alloy material by heating said powder solidified body and immediately
performing a warm extrusion process to it.